what about analog to digital converter ?
Submitted By: Erik Malund FAQ Last Modified: 12/01/06
- the most common mistake when using an A/D is confusing resolution with precision. It is easy to see a datasheet for a 24bit A/D and go WOW, I can make a precision measurement for 5 bucks - TOTALLY FALSE, the 24 bits is resolution, if you "throw the circuit together" you will, most likely, get a precision of about 8 bits.
A $10.000 laboratory precision scale will use a 24 bit A/D and thus have 24 bits of resolution, but reading the spec you will realize that it has only 16 bits of precision.
The name of the game is layout, layout, layout, this appnote http://www.linear.com/pc/downlo...1037,D4160 describes in detail how important this is. An experienced design team (decades of esperience) went through 28 layouts till they got a layout satisfactory for an economy scale, just visuailze the effort that goes into a precision scale.
Even with the most careful layout there will be some 'flicker' and thus averaging the readings is essential. It is common to average 8 or 16 readings simply because that gives the ability to divide by shifting which is much faster.
Even with averaging, you may see a bit of flicker, it will be less, but still confusing/annoying to the user. Thus it is common to apply a 'filter' i.e. only change the display if there is a dramatic difference and also update if a small difference has existed for a while. This gives instant response to daramatic changes and slow response to minuscule changes which most users find very pleasing.
Another issue is that in many designs you will need to add a "wild reading suppress" i.e. if a reading is far from the others in the "averaging stack" simply throw it away. That way if you have spurious noise it will not affect the readings. Re spurious noise, the "throw away" only works if the spurious noise is rare "throw away is not a "cure" for noise.
as a rule of thumb, I will suggest these obtainable precisions:
make a circuit: 8 bits precision
make a good circuit on a 2 layer board 9 bits
make a good circuit on a 2 layer board with ground fill 10 bits
make a good circuit on a 4 layer board 12 bits
spend 3-6 months making and testing layouts on a 6 layer board 14-16 bits
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